Device for narrow-band communication in a multi-carrier system

ABSTRACT

The invention relates to a central unit for a multi-carrier system, which comprises equipment for receiving a signal consisting of a plurality of carrier waves, and additionally comprises equipment for receiving a signal from equipment which transmits on one carrier wave, single-carrier equipment, in which this one carrier wave is a carrier wave that is part of the multi-carrier system for which the central unit is designed. The central unit is suitably equipped with means for scheduling the transmission from the single-carrier equipment in interaction with the single-carrier equipment.

TECHNICAL FIELD

[0001] This invention relates to a device which permits a narrow-banddevice, for example a hand-held computer, to communicate with a centralunit in a multi-carrier system in an energy-efficient way.

CURRENT TECHNOLOGY

[0002] A technique which is often used in modern digitaltelecommunication systems is so-called multi-carrier systems, in otherwords systems where a number of carrier waves at different frequenciesare used to transit information within one and the same time slot. Atypical multi-carrier system comprises a central unit that handles thecommunication with and between a number of subscribers within aparticular area. Such systems can also be used in, for example, officeenvironments, in so-called LAN-systems (Local Area Network).

[0003] The multi-carrier technique is a technique that demands arelatively high power supply for transmission, among other thingsbecause the amplifiers that are used in such systems have a lowefficiency rating. Many types of equipment used for wirelesscommunication in office environments, for example, portable computersand PDAs (Personal Digital Assistant) have power sources with extremelylimited capacity, which means that it is difficult to use such equipmentin multi-carrier systems. In spite of this, it is of course desirable tobe able to use, for example, portable computers and PDAs for wirelesscommunication also in systems where the multi-carrier technique is used.

DESCRIPTION OF THE INVENTION

[0004] The problem that is solved by the present invention is thus tomake it possible for equipment with a limited power supply to beincorporated in a multi-carrier system. The invention solves thisproblem by providing a central unit for a multi-carrier system, whichthus comprises equipment for the reception of a signal consisting of aplurality of carrier waves, which also comprises equipment for receptionof a signal with one carrier wave (single-carrier), where this onecarrier wave is one of the carrier waves that are included in themulti-carrier system for which the central unit is designed.

[0005] As transmission on a signal frequency can be carried out in amore energy-efficient way than multi-carrier transmission, in this wayit is possible for equipment that cannot communicate with the centralunit in a multi-carrier system on account of limitations in its powersupply, to be able to transmit to the central unit in spite of this bytransmission on a single frequency. The reception of multi-carriersignals does not require as much power as transmission, which means thatthe units with limited power can receive transmissions from the centralunit in a multi-carrier system in the same way as other units in thesystem.

[0006] The fact that single-carrier units in the system transmit on oneof the carrier waves that is included in the multi-carrier system forwhich the central unit is designed means that the modifications thatneed to be made to a central unit in order for it to be able to functionaccording to the invention are as small as possible.

[0007] In addition to being provided with equipment for the reception ofsignals on a single carrier wave, according to the invention the centralunit should be equipped with means for scheduling transmissions from asingle-carrier unit in interaction with this unit. The interaction withthe single-carrier equipment for the scheduling of its transmissionsshould suitably be carried out by modification of the existing dataframes that are used for scheduling communication within themulti-carrier system in which the central unit is incorporated.

[0008] According to the invention, single-carrier equipment is alsoprovided that is equipped with means for scheduling its owntransmissions to the central unit in interaction with a central unit ina multi-carrier system, preferably a central unit according to theinvention.

DESCRIPTION OF THE FIGURES

[0009] In the following, the invention will be described in greaterdetail, utilizing examples of preferred embodiments and with referenceto the attached figures, in which:

[0010]FIG. 1 shows the construction of different carrier-waves in amulti-carrier system, and

[0011]FIG. 2 shows a MAC frame in a known multi-carrier system, and

[0012]FIG. 3 shows communication over different carrier waves in adevice according to the invention, and

[0013]FIG. 4 shows a MAC frame in a system according to the invention,and

[0014]FIG. 5 shows a MAC frame in an alternative system according to theinvention.

EMBODIMENTS

[0015] The invention can be used in a large number of different types ofsystem that use different types of multi-carrier modulation. In thefollowing, the invention will be illustrated using such a system, theso-called Hiperlan/2 system, which uses so-called OFDM modulation,Orthogonal Frequency Division Multiplex.

[0016] OFDM in Hiperlan/2 and IEEE 802. 11 a uses 48 different carrierwaves for the transmission of data, as shown in FIG. 1. In addition tothe 48 carrier waves for data transmission, the system uses fourso-called pilot tones, which are shown in FIG. 1 by broken lines andindicated with arrows. The pilot tones can, for example, be used forsynchronization and measurement of signal strength. As also shown inFIG. 1, the 48 carrier waves are evenly distributed over the frequencyspectrum, except at the central frequency where there is no carrierwave.

[0017] Within the Hiperlan/2 system there are one or more central units,so-called Access Points (AP), each of which handles the communicationwith and between a number of subscribers, mobile terminals (MT), withina particular area. The area's APs tell each MT when it can receive datafrom the AP and when it can transmit data to the AP. All communicationin the Hiperlan/2 system is structured in time using so-called MACframes, Medium Access Control.

[0018]FIG. 2 shows the basic construction of a MAC frame within theHiperlan/2 system. When an MT wants to transmit data to AP, it iscarried out in the following way:

[0019] 1. MT sends a request to AP regarding being able to transmitdata. This takes place in RACH, Random Access Channel.

[0020] 2. AP sends out a signal that tells MT that its request regardingbeing able to transmit data has been received by AP. This is carried outin RFCH, Random Access Feedback Channel, in a later frame.

[0021] 3. If the request is accepted by AP, AP tells MT when MT cantransmit data to AP. This information concerning the allocatedtransmission slot is sent out in FCCH, Frame Control Channel, in anotherlater frame.

[0022] 4. MT transmits data to AP at the allocated time. This is carriedout in DU, Data Uplink.

[0023] In addition to the parts described above, the MAC frame alsocomprises DD, Data Downlink, in other words data from AP to MT, andBCCH, Broadcast Control Channel.

[0024]FIG. 3 shows schematically one of the principles behind theinvention. The system comprises a number, 1-N, of carrier waves. Anumber of these (in the Hiperlan/2 system four in number) are so-calledpilot-tones, which are intended, for example, for synchronization offrequencies.

[0025] In addition to the pilot tones, there is also a “gap”, with nocarrier wave, at the system's central frequency, the “zero frequency”.As the central frequency in the system is not used, and the pilot tonesare either unused or used to a minor extent, according to the inventionthese frequencies are used by one or more single-carrier units fortransmission to the central unit in the system.

[0026] In order to achieve the function according to the invention, boththe central unit and the single-carrier unit(s) should be provided withmeans for scheduling the transmissions from the single-carrier unit tothe central unit in interaction with each other. This is carried outaccording to the invention by the central unit partly having fullfunctionality to be able to communicate with the “ordinary” units in themulti-carrier system by means of the system's MAC frames, and also by itbeing provided with the ability to communicate with the single-carrierunits using modified MAC frames, as will be described below.Single-carrier units (MT) according to the invention have been given theability to communicate with the central unit (AP) using the modified MACframes, as will also be described below.

[0027]FIG. 4 shows an example of a modified MAC frame according to theinvention. The modification comprises the introduction of the abilityalso for single-carrier MTs to send a request regarding being able totransmit data, called RACH 2, where RACH 1 is the “ordinary” RACH thathas been described above in connection with FIG. 2. In addition the MACframe has been modified so that AP can also tell single-carrier MTs thattheir request regarding being able to transmit data (RACH 2) has beenreceived by AP, which is carried out in RFCH 2, where RFCH 1 is the“ordinary” RFCH that has been described above in connection with FIG. 2.

[0028] The single-carrier units suitably send their request regardingtransmission (RACH 2) on the central frequency, the “zero carrier wave”,but transmit their data on one of the pilot tones. This means that thesingle-carrier units can transmit to and receive from the AP at the sametime as the “ordinary” units, the multi-carrier units, in the system arecommunicating with the AP. In order for the single-carrier units to beable to transmit data to the AP on the pilot tones, the multi-carrierunits should not try to use the same pilot tones at the same time. Thiscan be arranged in two ways, either by changing the standard or by theAP in the system solving this by its scheduling of the transmission ofthe respective units.

[0029] The system is suitably arranged in such a way that not all thesingle-carrier units in the system send requests regarding transmissionat the start of RACH 2. This can be achieved by a number of techniquesthat spread out the transmission requests in time, for example,so-called “slotted ALOHA”, which techniques are familiar to experts inthe field.

[0030] In order for the single-carrier units in the system to know whenthey can transmit their requests regarding transmission, they must knowwhen RACH 2 commences in the relevant MAC frame. This information issuitably entered in each frame's FCCH, but can also be entered in theframe's BCCH.

[0031] Each unit in the system has its so-called “MAC-id”, which thecentral unit in the system uses to be able to identify information toand from the different units. If the information concerning the starttime for RACH 2 is entered in FCCH, the information can suitably beentered as a separate MAC-id. This MAC-id does not correspond to anyphysical entity, but is a MAC-id that the single-carrier units listenfor in order to obtain information from the central unit, for exampleconcerning the start and stop times for RACH 2.

[0032] Concerning the stop time for RACH 2, which coincides with thestop time for RACH 1, this can also be calculated by the single-carrierunits, based on the start time of the current frame, as all the MACframes have one and the same duration.

[0033] As shown in FIG. 4, the start time for RACH 2 should coincidewith the start time for DU, as this is the time when the central unitstarts to listen out for data from the units in the system.

[0034] As mentioned above, there is a plurality of different frequencieson which the single-carrier units in a system according to the inventioncan transmit data to the central unit in the system. In, for example,FCCH the central unit should therefore send out information to everysingle-carrier unit regarding which frequency that particular unit is totransmit data on at the time it has been granted permission to transmit,so-called “Resource Grant”. The information regarding which transmissionfrequency a particular unit has been allocated can be sent out in anumber of ways in FCCH, for example, by using unused bits in FCCH inorder to discuss this explicitly with the units, or by the unit's MAC-idbeing taken modulo N, where N is a predetermined number, for example thenumber of free frequencies that are available for transmission, in whichcase N will be equal to the number four in the Hiperlan/2 system.

[0035]FIG. 5 shows an alternative MAC frame that can be used in avariant of the invention. As shown in FIG. 5, this alternative MAC frameis identical to the one that has been described in connection with FIG.4, but with the difference that RFCH 2 is not included. In this variantof the invention, corresponding information is sent instead to thesingle-carrier units in FCCH, in other words information about the starttime for RACH 2 and information about whether the request regardingtransmission has been received by the AP. This information is sent usinga special MAC-id that is known by the single-carrier units, which MAC-idis reserved for this information. All information about all narrow-bandunits' RA feedback is under one and the same MAC-id, as a so-calledbitmap.

[0036] In order for the central unit in the system to be able to receiveboth multi-carrier signals and single-carrier signals at the same time,the central units according to the invention are modified in comparisonto traditional central units in multi-carrier systems. A suitable way ofcarrying out this modification is that the central unit's existingreceiver, that is to say the part(s) that handle(s) the conversion ofthe signal, is not changed, however a supplementary signal-processingfunction is introduced, the function of which is to separate the signalsthat arrive from the single-carrier units from the signals that arrivefrom the multi-carrier units. This supplementary signalprocessingfunction can be implemented in a large number of ways familiar toexperts in the field, and is not an essential part of the presentinvention, for which reason it is not described here in greater detail.

[0037] This invention is not limited to the embodiments described abovebut can be varied freely within the scope of the following patentclaims. Examples of variants within the scope of the invention are thatthe narrow-band units do not transmit or receive on a single frequency.Transmission and/or reception can be carried out in principle via anysubset of the frequencies that are available in the system.

1. A central unit for a multi-carrier system, comprising equipment forthe reception of a signal consisting of a plurality of carrier waves,additionally comprising equipment for the reception of a signal fromequipment that transmits on one carrier wave, a single-carrierequipment, whose one carrier wave is a carrier wave that is comprised inthe multi-carrier system for which the central unit is designed, thecentral unit also being equipped with means for scheduling thetransmission from the single-carrier equipment in interaction with thesingle-carrier equipment, characterized in that the interaction with thesingle-carrier equipment for the scheduling of the transmission from thesingle-carrier equipment is carried out by modification of the existingframe that is used for scheduling the communication within themulti-carrier system in which the central unit is comprised.
 2. Acentral unit according to claim 1 , in which the existing frame forcommunication within a multi-carrier system is modified in order to beable to receive requests (RACH) for data transmission from asingle-carrier unit.
 3. Single-carrier equipment, equipped with meansfor receiving transmissions in multi-carrier system technique, furtherbeing equipped with means for, in interaction with a central unit in amulti-carrier system, preferably a central unit according to any ofclaims 1-2, scheduling its own transmissions to the central unit,characterized in that the interaction with the central unit for thescheduling of transmissions is carried out by means of a modification ofone of the existing frames that are used for scheduling thecommunication within the multi-carrier system in which the central unitis comprised.
 4. Single-carrier equipment according to claim 3 , whichis equipped with means for sending requests for data transmission to thecentral unit.